1. Field of the Invention
The present invention relates to a software-driven time measuring device which is especially well suited for use in computer-containing control equipments involving accurate timing control, and in computer networks to manage operation timing among the computers, to set proper access time of the computers and to also permit shared use of the thus-set access time among the computers as needed.
2. Description of the Related Art
An atomic clock (cesium clock) is known at present as the most accurate time measuring device, which is accurate to one part in 10.sup.8 sec. Specifically, such an atomic clock defines, as one second, the duration of the natural resonance frequency of the cesium atom (9,192,631,770), and International Atomic Time is determined by the Bureau International de l'Heure (on the premises of the Paris Astronomical Observatory) by averaging the measured values of atomic clocks located throughout the world. The value of the second is thus managed today in accordance with the internationally determined atomic time, whereas the length of the day is managed in accordance with Universal Time. According to Universal Time, the hours of the day are numbered from 0 to 24, using as 0:00 p.m. (noon) a time point (southing time) when the sun crosses the Prime Meridian of longitude passing through the old Greenwich Observatory, England and using as 0:00 a.m. (midnight) a time point 12 hours before and after the southing time. The local standard time in each individual country of the world is set on the basis of a predetermined longitude passing through the country, and it is determined how many hours the local standard time is ahead or behind Universal time (Greenwich Mean Time). Specifically, Japan standard time is set, using as 0:00 p.m. a time point when the sun crosses Akashi Observatory (the 135th degree of east longitude). Further, in a large majority of the countries of the world, the Gregorian calendar is still used, in accordance with which each common year is set to have 365 days while every fourth year is set as a leap year having a total of 366 days. The Gregorian calendar was introduced on the basis of the fact that one revolution period of the earth relative to the sun (one solar year) is 365.2422 days, and it defines one year using its approximate value of 365.2425 days as one solar year.
However, the setting of the year and day based on the astronomical periods (such as the periods of the earth's revolution around the sun and rotation on its own axis) is not satisfactory, because the length of the day is somewhat changing due to the fact that the speed of the earth's rotation on its axis is not always constant by being influenced by fluctuations of the earth's axis and seasonal variations. In addition, because the speed of the earth's rotation on its axis has a tendency to slow down little by little, a slight difference arises between International Atomic Time constantly measured by the atomic clocks and Universal Time measured on the basis of the movements of heavenly bodies. This difference between the two times is currently compensated for by adding or removing one second (leap second) to or from the last minute on June 30 or December 31 in the year when it has exceeded 0.9 second.
The time management on the earth today is based on such Universal time and International Atomic Time, and various equipments existing on the earth, such as computer-containing control equipments involving accurate timing control, contain a time-keeping circuit (such as a quartz oscillator circuit), to which the current time (Universal Time) is input so as to perform timewise drive control of the equipments on the basis of time indicated by the time-keeping circuit.
In recent years, it has become necessary to remotely operate various control equipments loaded in a spacecraft operating off the earth's time space (such as a weather satellite moving around the earth and an interplanetary probe satellite), and to connect, in a network, computers located in various countries of the world so as to allow the computers to access information at predetermined timing. If, in such applications, time to be shared among the computers is set on the basis of Universal Time or the standard time of a specific country, a leap second occurring once in some years must be considered and proper access may not be guaranteed because it is unclear whether there exists a common time standard with another party's computer (e.g., whether a specific party's computer indicates the same time as the other party's computer). In view of this inconvenience, a variety of approaches have been proposed (e.g., in Japanese Patent Laid-open publication No. HEI 4-337943) to smooth the necessary time management, but they could not provide a satisfactory solution to the problem. Further, in the case of a spacecraft flying away from the earth to a far remote planet (such as the "Voyager" rocket searching Saturn), variations in gravitational field would cause "slowing of clocks" as referred to in Einstein's general theory of relativity even though a high-accuracy atomic clock is loaded in the spacecraft's computer. Namely, in a gravitational field far from the earth, electrons move more slowly and hence the frequency of radiated light becomes lower, so that the atomic clock measuring the frequency of light radiated from an atom (cesium atom) is unable to measure time accurately.